Method of fabricating radar chaff

ABSTRACT

The present invention is a method for making radar chaff consisting of metal rings having varying diameters, using photolithographic processes. One aspect of the invention has the steps of: providing a metal foil having a thickness suitable for use in radar chaff; overcoating the metal foil with a photoresist, to form a layered structure; undercoating the metal foil with a removable backing; exposing the photoresist to a quantity of actinic radiation for chemically modifying the photoresist into a photoproduct suitable for subsequent development, where the actinic radiation is patterned into a series of concentric rings; developing the photoresist, to expose the underlying metal in a series of concentric rings; removing the exposed metal to form a series of concentric metal rings on the removable backing; and removing the removable backing. Another aspect of the invention is a metal chaff precursor, comprising: a removable backing, coated with a plurality of concentric metal foil rings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to radar chaff, and more particularly toan efficient, low cost method of making radar chaff that completelycovers a wide bandwidth. This chaff can be efficiently packed fordispersal by standard dispersal techniques.

2. Description of the Related Art

Radar chaff is generally made from aluminum coated fiberglass which isbundled, cut into lengths, and packed for dispersal. Such chaff absorbsand returns radar most efficiently at frequencies which induce resonantoscillations of electromagnetic waves in the fiber lengths. Because alimited number of different segment lengths are packed into a bundle,the radar response to chaff is maximal at a relatively limited number offrequencies. For example, as shown in FIG. 1, a bundle 10 ofaluminum-coated glass fibers 12 may have several smaller bundles 14 ofseveral varying lengths, divided by separators 16, stacked fordispersal.

One possible means for packing a greater variety of lengths of chaffinto tubular containers is to use metal rings of varying diameters.Efforts have been made to fabricate metal rings of differing sizes bypunching them from metal foil or from thin plastic sheets coated with ametal film, but these attempts have not yielded rings of sufficientlywell controlled size to make them useful as chaff. The individual metalrings should preferably have thicknesses near 0.001", and have inner andouter radii differing by about the same amount (i.e., they should havecross sections of about 0.001"×0.001"). However, there is no reliablemethod for punching concentric rings of such fine cross sections. Also,when rings of larger cross-section have been made, great difficulty hasbeen experienced in efforts to stack the rings in arrays.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to produce radar chaff atlow cost.

It is a further object of this invention to produce radar chaff that hasan essentially continuous frequency response.

These and additional objects of the invention are accomplished by thestructures and processes hereinafter described.

The present invention is a method for making radar chaff consisting ofmetal rings having varying diameters, using photolithographic processes.One aspect of the invention has the steps of: providing a metal foilhaving a thickness suitable for use in radar chaff; overcoating themetal foil with a photoresist, to form a layered structure; undercoatingthe metal foil with a removable backing; exposing the photoresist to aquantity of actinic radiation for chemically modifying the photoresistinto a photoproduct suitable for subsequent development, where theactinic radiation is patterned into a series of concentric rings;developing the photoresists, to expose the underlying metal in a seriesof concentric rings; removing the exposed metal to form a series ofconcentric metal rings on the removable backing; and removing theremovable backing. Another aspect of the invention is a metal chaffprecursor, comprising: a removable backing, coated with a plurality ofconcentric metal foil rings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be obtained readilyby reference to the following Detailed Description of the PreferredEmbodiments and the accompanying drawings in which like numerals indifferent figures represent the same structures or elements, wherein:

FIG. 1 shows bundles of aluminum coated glass fibers used for radarchaff in the prior art.

FIG. 2 shows a top view of a plurality of concentric metal rings thatmay be used as radar chaff.

FIG. 3 shows a sectional view of a plurality of concentric metal ringsthat may be used as radar chaff.

FIG. 4 shows a sectional view of a stack of groups of concentric rings.

FIG. 5 shows a top view of a sheet having a plurality of groups ofconcentric metal rings on a common removable backing.

FIG. 6 shows a sectional view of a plurality of concentric metal ringson a removable plastic film backing.

FIG. 7 shows a sectional view of a plurality of concentric metal ringson a removable plastic film backing, where this plastic film includes asubstrate plastic layer and a degradable plastic layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, a group 20 of metal rings 22 in a concentricarrangement, may be made by a lithographic process according to theinvention. When backed by a removable backing 24, as shown in FIG. 3,this structure is a precursor for metal chaff according to theinvention.

Radar chaff preferably is made to resonate at a preselected frequency,corresponding to the frequency of some radar (e.g., x-band radar, IJband electronic warfare applications). For annular chaff, the resonantfrequency will be at a wavelength approximately equal to thecircumference of the chaff (λ=πD). If a group of metal rings is made sothat the rings are sufficiently close in diameter, the frequencyresponse of this chaff will be essentially continuous for wavelengthsover the range corresponding to the circumference range of the rings.Typically, at least 100 to 200 different ring diameters will be used ina group of rings used for radar chaff. For example, for a group of ringshaving widths of about 0.001", and spaces between these rings of about0.001", where the largest ring has a diameter of about 1.0", and thesmallest ring has a diameter of about 0.1", the frequency response ofthese rings will be essentially continuous over the range from about ν=4GHz to about ν=40 GHz. This size range may be scaled up or down,depending on the band of radar that this chaff is selected to defeat.Scaling down may be achieved simply by scaling down the lithography usedto make the rings. Scaling up may require additionally using astiffening layer, depending on the stiffness of the metal selected foruse in the chaff.

The process of the invention may be practiced in a variety of ways.Possibly the simplest way is to (1) coat a metal foil of an appropriatethickness with a photoresist; (2) expose the photoresist to patterned(typically by using a mask) actinic radiation (typically light) toconvert the photoresist to its photoproduct; (3) developing thephotoresist pattern with an appropriate developer; (4) undercoating themetal on the surface away from the resist with a lacquer such as anetch-resistant stop-off lacquer; (5) removing the metal exposed bydeveloping, typically by etching; and (6) removing the lacquer,typically by using a lacquer solvent such as acetone. Good results havebeen achieved with Miccroshield™ Stop-off lacquer (Pyramid Plastics).

If, as shown in FIG. 4, it is desired to stack the groups 20 ofconcentric rings 22 in a large stack 40, preferably the lacquer isremoved after the groups of rings are stacked. Otherwise, it will bedifficult to stack a large group of loose rings.

FIG. 5 shows a too view of sheets having a preferred arrangement ofgroups of concentric metal rings on a common removable backing. Aplurality of these sheets may be stacked, providing a preferred way ofsimultaneously stacking a number of groups of concentric rings.

Another method of practicing the invention uses a plastic film insteadof a lacquer as the removable backing. As shown in FIG. 6, a group 20 ofconcentric metal rings 22 may be disposed over a removable plastic film62. Polyacetyl, which may be removed by exposure to nitric acid fumes,is a preferred removable plastic film. Preferably, this plastic film 62will have a central channel 64 for improving contact between theremovable plastic film and the removing agent. Typically, this channel64 will be a punched hole.

In another preferred embodiment of the invention, shown in FIG. 7, thisplastic film 62 is a bilayer plastic film including a substrate plasticlayer 66 and a degradable plastic layer 68. The substrate plastic layer66 is stable under conditions that will degrade (and thus remove) thedegradable plastic layer 68. Preferably, the substrate plastic layer 66is non-adherent to the metal 22 or to another substrate plastic layer66. Preferably, the substrate plastic layer 66 is not attacked by anetchant used to remove exposed metal. Preferred substrate plastic layersinclude teflon and polyethylene.

Thus, a method for practicing the invention using a plastic film as theremovable backing has the steps of: coating a plastic film with a metalfoil; overcoating the metal foil with a photoresist to form layeredstructure; exposing the photoresist to a quantity of actinic radiationfor chemically modifying the photoresist into a photoproduct suitablefor subsequent development, where the actinic radiation is patternedinto a series of concentric rings; developing the photoresist, to exposethe underlying metal in a series of concentric rings; and removing theexposed metal to form a series of concentric metal rings on the plasticfilm.

In another preferred embodiment of the invention, prior to the step ofovercoating the metal foil with photoresist, the metal foil isovercoated with a ceramic layer. Preferably, the ceramic layer iselectrostatically charged, to aid in dispersing the metal rings byelectrostatic repulsion. Preferred ceramics include oxides, carbides,nitrides, and borides.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A method for making radar chaff comprising metalrings having varying diameters, comprising the steps of:providing ametal foil having a thickness suitable for use in radar chaff;overcoating said metal foil with a photoresist, thereby forming alayered structure; undercoating said metal foil with a removablebacking; exposing said photoresist to a quantity of actinic radiationfor chemically modifying said photoresist into a photoproduct suitablefor subsequent development, wherein said actinic radiation is patternedinto a series of concentric rings; developing said photoresist, therebyexposing said underlying metal in a series of concentric rings; removingsaid exposed metal to form a series of concentric metal rings on saidremovable backing; and removing said removable backing.
 2. The method ofclaim 1, wherein said removable backing comprises an etch-resistantlacquer, and said step of removing said removable backing comprisescontacting said removable backing with a solvent for said etch-resistantlacquer.
 3. The method of claim 1, wherein said step of undercoatingsaid metal foil is conducted after said steps of overrcoating said metalfoil with a photoresist, exposing said photoresist, and developing saidphotoresist.
 4. The method of claim 1, wherein said step of undercoatingsaid metal foil is conducted before said step of overrcoating said metalfoil with a photoresist.
 5. The method of claim 1, wherein said step ofundercoating said metal foil with a removable backing comprises coatinga removable plastic film onto said metal foil.
 6. The method of claim 5,wherein said step of exposing said photoresist to patterned actinicradiation comprises exposing said photoresist to light through a maskpatterned with a series of concentric rings.
 7. The method of claim 5,wherein said step of removing said metal comprises etching away saidmetal.
 8. The method of claim 5, further comprising the step of:openinga channel through said layered structure, normal to the plane of saidlayered structure, at about the center of said concentric rings.
 9. Themethod of claim 5, wherein said plastic film comprises polyacetyl. 10.The method of claim 5, further comprising the step of:exposing saidplastic film to nitric acid fumes after said step of removing saidexposed metal, thereby removing said plastic film.
 11. The method ofclaim 5, wherein said plastic film comprises a substrate plastic layerand a degradable plastic layer interposed between said substrate plasticlayer and said metal, wherein said substrate plastic layer is stableunder conditions that will degrade said degradable plastic layer. 12.The method of claim 11, further comprising the step of:removing saiddegradable plastic layer from said metal rings, thereby forming metalchaff.
 13. The method of claim 11, wherein said substrate plastic layeris nonadherent to said metal foil or to another substrate plastic layer.14. The method of claim 11, wherein said substrate plastic layer isselected from the group consisting of teflon and polyethylene.
 15. Themethod of claim 5, further comprising the step of:prior to said step ofovercoating said metal foil with said photoresist, overcoating saidmetal foil with a ceramic layer.
 16. The method of claim 15, whereinsaid ceramic layer comprises a ceramic selected from the groupconsisting of oxides, carbides, nitrides, and borides.
 17. The method ofclaim 15, wherein said ceramic layer is electrostatically charged.
 18. Amethod for making radar chaff consisting of metal rings having varyingdiameters, comprising the steps of:(a) coating a plastic film with ametal foil; (b) overcoating said metal foil with a photoresist, therebyforming a layered structure; (c) exposing the photoresist to a quantityof actinic radiation for chemically modifying said photoresist into aphotoproduct suitable for subsequent development, wherein said actinicradiation is patterned into a series of concentric rings; (d) developingsaid photoresist, thereby exposing said underlying metal in a series ofconcentric rings; (e) removing said exposed metal to form a series ofconcentric metal rings on said plastic film; (f) repeating said steps(a) through (e) a plurality of times, thereby forming a plurality ofpatterned layered structures; and (g) coaxially stacking said pluralityof said patterned layered structures.
 19. A metal chaff precursor,comprising:a removable backing, coated with a plurality of concentricmetal foil rings.
 20. The metal chaff precursor of claim 19, whereinsaid concentric metal foil rings have circumferences selected tocorrespond to the wavelengths of selected radars.
 21. The metal chaffprecursor of claim 19, wherein said concentric metal foil rings havecircumferences ranging from about 1.0" to about 0.1".
 22. The metalchaff precursor of claim 19, wherein said concentric metal foil ringshave cross sections of about 0.001"×0.001".